1. Field of Invention
The present invention relates to a motor control system provided with a mechanical brake which stops an output axis of a servo motor once the excitation of the servo motor is released, and especially relates to a motor control system provided with a function of detecting abnormalities in such a mechanical brake.
2. Description of the Related Art
A machine tool which moves a main spindle head along a ball screw by rotating the ball screw by a servo motor is conventionally known. A tool such as an end mill or drill is attached to the main spindle head, and a workpiece is machined by moving the main spindle head towards the workpiece while rotating the tool.
Further, there is a machine tool, such as a vertical milling machine, wherein a ball screw extends in the direction of gravity, so that the main spindle head is moved up and down in the direction of gravity. In such a machine tool, if the excitation of the servo motor is released when the power is off or during an emergency stoppage, the ball screw can rotate freely and there is a risk that the main spindle head may fall due to gravity. Therefore, in a machine tool where the main spindle head moves up and down, a mechanical brake for stopping the output axis of the servo motor when the power is off or during an emergency stoppage, is attached.
Further, in an industrial robot, a servo motor is used to drive an axis of the robot, and therefore, there is also a risk that the arm of the robot will fall when the excitation of the servo motor is released when the power is off or during an emergency stoppage. Therefore, in industrial robots a mechanical brake for stopping the output axis of the servo motor is mounted.
Further if abnormality exists in the aforementioned mechanical brake, when the power is off or during an emergency stoppage, the main spindle head of the machine tool or the robot arm may fall. Accordingly, as illustrated in Japanese Patent No. 3081258 or Japanese Patent Application Laid-open No. H06-284766 or the like, a method for detecting abnormalities in the aforementioned mechanical brake has been proposed.
Japanese Patent No. 3081258 discloses a mechanical brake including a brake disc connected to an output axis of a motor, and a brake shoe pressed against the brake disc. Further, in a method of detecting a failure of the mechanical brake disclosed in Japanese Patent No. 3081258, when braking a rotary axis of the motor, the current value at the start of the motor rotating is detected while the current supplied to the motor is gradually increased. Similarly, when the rotary axis of the motor is not braked, the current value at the start of the motor rotating is detected while the current supplied to the motor is gradually increased. Further, by comparing the difference between the detected current values and a set value corresponding to the brake torque, a failure of the mechanical brake is detected.
Moreover, in the method of detecting abnormality in a mechanical brake disclosed in Japanese Patent Application Laid-open No. H06-284766, when the amount of wear of a friction braking member of a mechanical brake is within a normal range, the motor is driven without actuating the mechanical brake and the motor load current value at the time the motor is driven is measured and stored beforehand. In the case inspecting the mechanical brake, as when the amount of wear of the friction braking member is within a normal range, the motor is driven without actuating the mechanical brake, and the size of the motor load current while the motor is driven is detected. Further, abnormality of the mechanical brake is detected by comparing the detected motor load current value with the previously stored motor load current value.
Further, any abnormality detected in the aforementioned mechanical brake is indicative of a significant reduction in the force for stopping the output axis of the servo motor. Thus, there is a risk that the main spindle head of the machine tool or the robot arm may fall unless the inspection or repair of the mechanical brake is completed. To address this, it is desired that the shortage of the braking force for the output axis of the servo motor be compensated by some kind of method immediately after the detection of the abnormality of the mechanical brake. To exemplify such a method, Japanese Patent Application Laid-open No. 2013-032825 discloses a power transmission controller of a vehicle, wherein, in the case of abnormality in a brake, a new braking force is generated in addition to a friction braking force to ensure a sufficient braking force.
The mechanical brake abnormality detection methods disclosed in the aforementioned Japanese Patent No. 3081258 and Japanese Patent Application Laid-open No. H06-284766 are methods in which the diagnosis of the existence of abnormality of the mechanical brake is carried out by separately executing a mechanical brake inspection program. Accordingly, in the abnormality detection method for a mechanical brake disclosed in Japanese Patent No. 3081258 and Japanese Patent Application Laid-open No. H06-284766, there is the problem that there are cases when abnormality has occurred in the mechanical brake before the inspection of the mechanical brake is carried out. Therefore, it is desirable to have a method of detecting abnormality of a mechanical brake or indications thereof even when an inspection of the mechanical brake is not carried out.
Moreover, there is the problem that the power transmission controller of a vehicle disclosed in Japanese Patent Application Laid-open No. 2013-032825, in which a plurality of clutches are used in a mechanism compensating for the shortage of a friction braking force, has a structure too complicated for application to the output axis of the servo motor.